VECTOR: Star Castle Background Sound Problem.

Hi Vectorlist,

I could use some help with a problem that has me pretty much vexed at the
moment. I'm doing my best with a logic probe and a digital multimeter.

Symptom: Background and star sound has a brief pause when it oscillates.

I'm pretty sure I have it nailed as a problem with the 7406 open collector
inverter. It is receiving pulses from the LS377 at IC3, but there is no
output from respective pins.

I know that with this type of IC that there needs to be voltage present in
the circuit at the outputs in order for the 7406 to invert the incoming
signal and ground each output. If I ground out each output pin, I hear
sounds generated of different pitches.

Now, to make a long story even longer, I've tested each and every resistor
and diode in the DAC circuit and even replaced the TL081s. I've tested all
the voltages at the outputs of the 7406, everything is extremely close to 5
(4.9 - 5.02) volts except for the output at pin 12 which is 3.9. I suspect
that this isn't high enough to pull up the proper outputs and is causing the
chip not to operate as it should. 

Any help would be greatly appreciated!

Best,

Jim